Is the profit after crushing waste plastic high? At present, plastics are mainly polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP) and polystyrene (PS), PE agricultural films, PS packaging materials, PP woven bags, PVC sheets, plates, pipes And special materials are plastics with a particularly large amount of waste. The pyrolysis of waste plastic is similar to the pyrolysis of municipal waste, but the pyrolysis products of waste plastic are mainly fuel oil or chemical raw materials with higher added value. It can be said that the pyrolysis and oilification of waste plastics is the reverse process of manufacturing plastic products in the petrochemical industry using petroleum as the raw material. The pyrolysis of waste plastics reduces plastics originally made of petroleum to petroleum products, which can effectively recover energy.
Characteristics of waste plastic pyrolysis:
A The thermal decomposition characteristics of plastics include many types of plastics, mixed types of waste plastics, and difficult sorting. Due to the different composition of plastics, their cracking behavior and characteristics are different. The thermal decomposition of various plastics is shown in Figure 5-68. Polyethylene, polypropylene and polystyrene decompose almost completely at 300 ~ 400 ℃. Polyvinyl chloride decomposes in two stages at 200 ~ 300 ℃ and 300-400 ℃, and releases HC gas. Because HCI gas is severely corrosive to the reaction equipment, and it affects the service life of the cracking catalyst and the quality of diesel and gasoline, therefore, it is generally required that the cracked raw materials do not contain PVC waste plastic.
At present, the thermal decomposition reaction temperature of used plastics is often higher than 600 ° C, which mainly produces olefins and a small amount of aromatic hydrocarbons. When the heating rate is fast or the residence time is short, a large amount of ethylene and propylene can be produced. In the presence of water vapor, olefin production can also be significantly increased. Recently, there are also a few examples of thermal decomposition reaction at 500 ° C. The reaction products include linear alkanes, olefins, and a small amount of aromatic hydrocarbons. When the content of chlorine in the reaction product is less than 10x10-, the product can be further purified by hydrogenation.
B waste plastic thermal decomposition characteristics
The thermal conductivity of waste plastic is lower 0.08-0.35 w / (m- ℃) (equivalent to dry wood), when heated to the melting point temperature (100 ~
250 ℃), the core temperature is still very low, continue to heat, the external temperature can reach above 500 ℃ and carbonization, but the internal temperature can reach the degree of melting. External carbonization hinders internal decomposition, resulting in low thermal efficiency.
Thermal decomposition types of plastics can be divided into depolymerized reaction type plastics and randomly decomposed type plastics, as well as intermediate parts, which are both decomposed type plastics. When the depolymerization reaction type plastic is thermally decomposed, the polymer dissociates and decomposes into monomers, which mainly cuts the bond between monomer molecules. Such plastics include polyoxymethylene, poly-a-methylstyrene, polymethyl methacrylate, tetrafluoroethylene plastic, etc., and they are almost 100% decomposed into monomers.
When the random decomposition type plastic is thermally decomposed, the chain break is random, so a low-molecular compound without a certain number of carbon atoms and hydrogen atoms is combined. Such plastics include polyethylene and polyvinyl chloride.
Most plastics decompose on heating, and both have both. The proportion of various decomposition products varies with the type of plastic and the decomposition temperature. Generally, the higher the temperature, the higher the proportion of gaseous (lower) hydrocarbons. Due to the complex product components, it is difficult to decompose various individual components. Generally, only gaseous, liquid, and solid components are used for recycling. In addition, there are types of pyrolysis that use incomplete combustion of plastic to recover carbon black.
Plastics containing fluorine and chlorine groups, thermal decomposition products generally contain HCI and HCN, while plastic products contain less sulfur and thermal decomposition: the resulting oil contains a correspondingly low bowl content, which is a high quality low Sulfur fuel oil.
c waste plastic thermal decomposition products
Thermal decomposition products also differ greatly depending on the type of plastic. In general, thermal decomposition reactions can generate four types of reaction products: hydrocarbon gases (the number of carbon molecules is GG), and oil products (the number of carbon molecules of gasoline is Cs-Ci. The number of carbon molecules of diesel is C2 ~
C20, carbon number of heavy oil is greater than Ca), paraffin and coke. The output of different reaction products mainly depends on the type of plastic, reaction conditions, type of pyrolysis furnace and operation method. Polystyrene PS and polymethyl methacrylate PMMA mainly generate corresponding monomer substances, while the reaction products of other types of plastics are relatively dispersed. The reaction temperature is a key factor affecting the thermal decomposition reaction products. When the reaction temperature increases, the gas and coke production increases, while the oil production decreases. Figure 5-69 shows the relationship between the thermal decomposition depth of plastics and the reaction products. The thermal decomposition rate of polyolefin plastics is related to the branch and side chain substituents. High-density polyethylene (HDPE) has a pyrolysis temperature of 447 ° C, low-density polyethylene (LDPE) has a pyrolysis temperature of 417 ° C, polypropylene (PP) has a pyrolysis temperature of 407 ° C, and polystyrene
(PS) is 376 ° C. When polyvinyl chloride (PVC) plastic is thermally decomposed, HCI is released at a lower temperature (200-360 ° C) to produce multiple hydrocarbons, and then further decomposes at a higher temperature. The order of thermal decomposition speed is HDPE <LDPE <PP <PS. Different plastics have very different molecular structures, so the composition and recovery of pyrolysis products are also different. Polystyrene
(PS), polyethylene (PE), polypropylene (PP), polybutene (PB), etc., are easily thermally decomposed at 400 ~ 500 ° C to produce light oil.
Especially PS, PE.PP and APP, the thermal decomposition performance is very good, the oil yield can reach 80% ~ 90%, and the quality of the produced oil is also very high.
Because these polymers have a simple pyrolysis process and a high oil yield when the raw materials are single, Table 5-8 shows the thermal decomposition recovery rates and the composition and content of thermal decomposition products of several typical plastics. Therefore, if conditions permit, separate recycling and pyrolysis of plastics of different materials as much as possible to reduce investment costs and operating costs, and improve economic efficiency.
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